Most environmental processes, such as the water and carbon cycles, are based on large-scale circular principles since materials flow from one state to another. However, true circular principles are impossible from an entropic perspective since environmental processes require an important external input, which is energy (solar), to function. The circular economy tries to imply some of these principles on a smaller scale, such as within definable supply chains. Similar to environmental processes, a circular economy cannot function without large and low-cost energy inputs.